In multi-pickup tube color TV cameras utilizing magnetically focused pickup tubes, it is essential for registration reasons to maintain precise control of several associated pickup tube parameters. These include: focus coil mechanical configuration, focus coil current, and G-3/G-4 voltage ratio of each pickup tube where G-3 represents the focus cylinder voltage and G-4 represents the field mesh voltage. To do all of these in a cost effective fashion is the concern of the subject invention.
Each pickup tube in a multi-pickup tube camera system contains a focus cylinder (G-3) and a field mesh (G-4). The ratio of the voltages applied to these two electrodes affects tube performance characteristics such as beam landing error, center-to-corner focus, and geometry of the reproduced image. Electronic focusing of the pickup tube scanning beam may be achieved by either: (a) adjusting the G-3 and G-4 voltages in concert such as to retain a constant ratio; or (b) adjusting the focus coil current. It is desirable to provide remote focusing control of the camera by a digital control system using microprocessors. The G-3/G-4 voltages are of the order of 1,000 volts and the precision required is such as to make it impractical to derive these digital signals from a low voltage control such as associated with a digital control system. The focusing of each tube may be accomplished by individual current control for each focus coil. A disadvantage of this type of system is that there can exist differential shifts in the focus coil currents resulting in registration errors.